The present application relates to the field of batteries. More particularly, the present application relates to batteries having auxiliary electrodes that are intended to provide improved capacity fade performance.
Lithium-ion batteries or cells include one or more positive electrodes, one or more negative electrodes, and an electrolyte provided within a case or housing. Separators may be provided between the electrodes to prevent direct contact between adjacent electrodes. The positive and negative electrodes each include a current collector having an active material provided thereon. The active materials of the positive and negative electrodes may be provided on one or both sides of their respective current collectors.
FIG. 1 shows a schematic representation of a portion of a lithium-ion battery 10 such as that described above. The battery 10 includes a positive electrode 20 that includes a positive current collector 22 and a positive active material 24, a negative electrode 30 that includes a negative current collector 32 and a negative active material 34, an electrolyte material 40 disposed generally between the positive electrode 20 and the negative electrode 30, and a separator (e.g., a polymeric microporous separator; not shown) provided between the positive electrode 20 and the negative electrode 30. The electrodes 20, 30 may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an ovular or circular cylindrical configuration). The electrodes may also be provided in a folded configuration.
As shown in FIG. 1, during charging and discharging of the battery 10, lithium ions move between the positive electrode 20 and the negative electrode 30. For example, when the battery is discharged, lithium ions flow from the negative electrode 30 to the positive electrode 20. In contrast, when the battery 10 is charged, lithium ions flow from the positive electrode 20 to the negative electrode 30.
FIG. 4 is a graph 400 illustrating the theoretical charging and discharging behavior for a conventional lithium-ion battery having a positive electrode with an aluminum current collector and LiCoO2 active material and a negative electrode with a copper current collector and a carbonaceous active material. Curves 410 and 420 represent the positive electrode potential and the negative electrode potential, respectively, versus a lithium reference electrode. The difference between the curves 410, 420 represents the overall cell voltage.
With reference to FIG. 4, during initial charging to full capacity, lithium ions are transferred from the positive electrode to the negative electrode. The potential of the positive electrode moves rightward along curve 410, increasing from approximately 3.0 volts to approximately 3.8 volts. The potential of the negative electrode moves rightward along curve 420, decreasing from approximately 2.0 volts to approximately 0.2 volts. Also during the initial charge, due to the formation of a passive layer on the negative electrode (i.e., a solid-electrolyte interface (“SEI”)), the negative electrode experiences an irreversible loss of capacity (i.e., gains irreversible capacity). The irreversible capacity is represented by the shelf 424.
During a subsequent discharge, lithium ions are transferred from the negative electrode to the positive electrode. The potential of the positive electrode moves leftward along curve 410. The potential of the negative electrode moves left ward along curve 420 and, then, along dashed curve 426, until leveling off at approximately 3.5 volts (i.e., the corrosion potential of the copper negative current collector). The positive and negative potentials then meet at the crossing potential 442, where the battery is at approximately zero volts (i.e., deep discharge).
One difficulty with conventional lithium-ion batteries is the instability of the positive and negative electrodes in deep discharge conditions, because the current collectors may corrode or the active materials may decompose at certain crossing potentials. For example, referring again to FIG. 4, if the crossing potential is at or above approximately 3.5 volts (the approximate corrosion potential 450 of copper) the copper negative current collector may corrode, or a graphitic negative active material may decompose. If the crossing potential is at or below approximately 1.8 volts, the LiCoO2 positive active material may decompose, or if below 0.3 volts, the aluminum positive current collector may corrode. Over time, degradation of the current collectors and active materials results in decreased ability of the positive and negative electrodes to be doped and undoped with lithium (i.e., store and transfer lithium). Accordingly, battery capacity is lost over time (i.e., the battery experiences capacity fade).
Further, the instability of battery electrodes and capacity fade occurring in deep discharge conditions is compounded by unpredictable shifts in crossing potential caused by changes in relative irreversible capacity of the positive and negative electrodes. For example, referring again to FIG. 4, if the positive electrode were to experience higher irreversible capacity loss than the negative electrode, a negative crossing potential would result (i.e., a crossing potential at or near the average potential of the negative electrode). During discharge, the potential of the positive electrode would move leftward along curve 410 until becoming fully saturated with lithium (i.e., before the negative electrode may become fully depleted), and the potential of the positive electrode would rapidly approach the potential of the negative electrode 420 to achieve a negative crossing potential (i.e., at the average potential of the carbonaceous negative active materials). This crossing potential would be below the decomposition potential 460 of the LiCoO2 positive active material and would, thus, cause the positive active material to decompose and the battery to lose capacity.
Accordingly, it would be advantageous to provide a lithium-ion battery with increased resistance to capacity fade and a controlled and well-defined crossing potential for the battery.
Further, the medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with drug therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life.
It may be desirable to provide a source of battery power for such medical devices, including implantable medical devices. In such cases, it may be advantageous to provide a battery that may be recharged. It may also be advantageous to provide a battery that may be discharged to a near zero voltage condition without substantial risk that the battery may be damaged (e.g., without corroding one of the electrodes or the battery case, decomposing the positive active material, etc.) such that performance of the battery is not degraded in subsequent charging and discharging conditions.